The present disclosure relates generally to control methods and apparatuses relevant to TRIACs, more specially to control methods and apparatuses for providing the holding current that a TRIAC needs for proper operation.
TRIAC dimmers are designed for resistive loads such as incandescent or halogen lights, and they have been significantly installed in the United States and worldwide. Unfortunately, these phase-controlled dimmers are not readily compatible with LEDs since LEDs do not appear as a resistive load. Therefore LED-based solutions using traditional LED drivers will not perform as expected with TRIAC wall dimmers.
A TRIAC dimmer blocks or cuts off a portion of the waveform of an alternating-current (AC) voltage power source, so as to reduce the power transferred through and to dim the light source that the TRIAC drives. Therefore, TRIAC dimmers are also named as phase-cut dimmers. FIGS. 1 and 2 demonstrate waveforms generated by a leading-edge TRIAC dimmer and a trailing-edge TRIAC dimmer, respectively. Waveform 100 represents the voltage of an AC outlet connected to a power grid. The shadowed areas 110 mean the portions of the waveform 100 that a TRIAC dimmer bypasses to a load. A cutoff time TCUTOFF refers to a period of time when a TRIAC dimmer blocks the AC voltage of the power grid; and a conduction time TCONDUCTION refers to a period of time when a TRIAC dimmer just bypasses the AC voltage to a load.
As shown in FIGS. 1 and 2, the voltage across a load during a cutoff time TCUTOFF is almost 0V. Nevertheless, a load must drain a minimum amount of current during a cutoff time TCUTOFF to keep a TRIAC dimmer work properly, and this current is called holding current in the art. When the holding current becomes zero, a TRIAC dimmer restarts or resets. The holding currents for different TRIAC dimmers differ, depending on their designs and specifications.
It is a challenge for power converter manufactures to design a LED driver capable of providing a holding current during the cutoff time TCUTOFF.